1. Field of the Invention
The present invention relates to a method for preparing a magnetic recording medium and more particularly it relates to a method suitable for preparing a magnetic recording medium having a thin magnetic layer of dry thickness of 2 .mu.m or less.
2. Background of the Invention
Generally, a magnetic recording medium is prepared by coating a belt-shaped non-magnetic support with a coating solution containing a dispersion of ferromagnetic particles and a binder dissolved by a solvent while the support is continuously being run in its length-direction. The coating solution is dried and solidified and the support is slit.
However, upon preparing a magnetic tape, in order to increase sensitivity and to improve the S/N ratio, it is necessary that ferromagnetic particles be aligned in the travelling direction of a non-magnetic support, thereby increasing a squareness ratio (the ratio of the residual magnetization B.sub.r to the saturation magnetization B.sub.m) of the magnetic layer. Accordingly in a conventional method, upon preparing a magnetic tape, the axis of easy magnetization of ferromagnetic particles is set in the travelling direction of a non-magnetic support by orienting ferromagnetic particles in the travelling direction with permanent magnets, a solenoid or the like while the coating solution remains undried.
Recently, the increase of large size memories and the increase of recording and reproducing performance in a magnetic recording medium such as a magnetic disk or a magnetic tape have strongly been desired.
In order to increase memory capacity, it is, needless to say, necessary to increase the recording information density per unit area of a magnetic recording medium. On the other hand, in order to increase recording information density, magnetic flux coming from a magnetic head must be concentrated to an extremely small area. For this reason, a magnetic head must be miniaturized and the quantity of magnetic flux must be decreased.
In order to reverse the direction of magnetization with such a small reduced quantity of magnetic flux, it is necessary to decrease the volume of a magnetic layer. Accordingly, satisfactory magnetization inversion cannot be carried out unless the thickness of a magnetic layer is decreased.
For the above reason, it becomes necessary to decrease the thickness of the magnetic layer.
Furthermore, in order to increase output signals for recording and reproducing, it is necessary to increase residual magnetic flux of a magnetic layer. For this purpose, the thickness of a magnetic layer must be increased, but when the thickness thereof is increased, the high frequency characteristics deteriorate. In this connection, in order to increase residual magnetic flux and to improve high frequency characteristics, it is necessary not only to decrease the thickness of the magnetic layer but also to increase the coercive force of magnetic materials. In order to meet the above requirements, it is suggested that magnetic materials having high coercive force are used to prepare a thin magnetic layer. In this case, however, high frequency characteristics are improved but low frequency characteristics are degraded. Furthermore, when the thickness of a magnetic layer prepared from magnetic materials having high coercive force is increased, a high frequency magnetic field penetrates only on the surface of the magnetic layer. Therefore, it is difficult to perform erasure.
To solve the contradictory problems as above, an ideal magnetic recording medium is produced which has a comparatively thick lower layer prepared from magnetic materials having comparatively low coercive force and high residual magnetic flux and a thin upper magnetic layer provided on top thereof prepared from magnetic materials having high coercive force. In this way, a method of coating two layers separately and conducting magnetic orientation has conventionally been conducted.
However, when two layers are provided and the dried thickness of the upper thin magnetic layer is 2 .mu.m or less, it was found that the squareness ratio is not remarkably improved.
It is reasoned that due to such a thin upper magnetic layer of 2 .mu.m or less, drying speed becomes so fast that the viscosity of the layer increases rapidly before the magnetic field is formed. As a result, ferromagnetic particles move with difficulty.
It is furthermore reasoned that when a second thin magnetic layer is provided on the first dried magnetic layer, solvents used in the second layer are adsorbed by the first layer, thereby increasing the viscosity of the second layer more rapidly than ever in addition to the above phenomenon. And as a result, ferromagnetic particles move with difficulty.